THE HORIZONTAL, REGULARLY-ORIENTED STRUCTURES

The morphological uniqueness of these structures is related to their repeated, uniform size and shape, as well as their strictly horizontal and aligned appearance (Figure 2). While the most spectacular occurrences are those where the density is more than one hundred structures per meter square along well-exposed horizontal surfaces of up to 100 m² (Figure 2.1, 2.2), the structures are found in relative isolation as well (Figure 2.3). Individual structures can be followed for 50 to 150 mm; however, it is not clear whether their terminations are natural or due to weathering. In vertical exposures (Figure 2.4, 2.5), the diameters of these unbranched, cylindrical (tube-like) and virtually straight structures vary only slightly from 3 to 5 mm. The diameter is maintained over the entire length of the individual structure (hence the cylindrical shape), and within a given exposure. The structures show no evidence of overlapping or crosscutting one another, and show no external sculpture or internal grain arrangements. The structures are preserved both in full relief (Figure 2.4, 2.5) within the host rock, and positive (convex) relief on flat surfaces (epirelief) (Figures 2.1-2.3, 2.5, Figure 5.3). These flat surfaces are difficult to classify as bedding planes or weathering surfaces. Nonetheless, it is clear that the inclinations of these surfaces are parallel to the bedding planes of the over- and underlying strata. The zones containing full relief structures are variable in thickness, ranging from a few centimeters up to 0.6 m (Figure 2.4, 2.5 Figure 5.3).

The long axes of these structures are invariably parallel to one another and to the major horizontal bedding planes in the surrounding stratigraphic units. This spatial relationship of the structures is consistent throughout the studied outcrops in southern Africa; however directional scattering of the mean vectors is evident laterally between outcrop areas (Figure 5.1). The most common orientations are north-northwest - south-southeast (40% of the measurements), and north-northeast - south-southwest (30% of the measurements) but east-west oriented structures are also present in one locality (Figure 5.2). Furthermore, at localities 3 and 6 (Figure 5.1), the mean vectors of the horizontal, regularly-oriented structures found in two vertically successive zones occur at right angles to one another (see Figure 5.3 and star markings on Figure 5.2). Note that at Locality 3, the structures occur towards the upper part of the Clarens Formation, approximately 20 m below the first Karoo flood basalts. Owing to the massive appearance of the host rock, there is no indication of any obvious stratigraphic gap (e.g., erosion surface, discordant strata) between the consecutive layers containing the differently oriented horizontal structures.

While in most occurrences, the outlines of the structures are well-defined due to differential weathering of the massive infilling and surrounding host rock (Figures 2.3, 2.5, Figure 5.3), in some outcrops, the true outline of the tubes is poorly defined macroscopically as they are surrounded by an indurated zone of well-cemented sediment which causes the structures to appear much larger (e.g., up to 25 mm in diameter) than it is. In fact, some of the structures shown in Figure 2.1-2.2 comprises two or three structures that are fused together by a zone of stronger cementation, and thus form thicker elongated features. The much smaller, true diameter of these weathered specimens is clearly visible in cross-sectional views (Figure 2.4, 2.5). That is especially true upon magnification, where the structures become even more evident because of textural discontinuities in form of extremely subtle, ~0.1 mm thin gaps surrounding each of the ~5 mm diameter cylinders (Figure 6). This small gap separates the massive infilling of the structures from the surrounding host rock, both of which comprise the same, moderately to well-sorted, predominantly fine-grained (0.2 mm), massive, silica-cemented, quartz-arenite, which contain mainly subrounded and subangular, monocrystalline quartz grains, <1% feldspar, and a few chert fragments.